1. Field of the Invention
The present invention relates to an NMR probe using a variable tuning frequency and, more particularly, to an NMR probe equipped with a built-in disk-like tuner block having tuning elements (such as inductors, capacitors, and other elements forming a resonant circuit) on its both faces.
2. Description of Related Art
FIG. 1 shows main portions of a conventional NMR spectrometer. This NMR spectrometer is used for multiple resonance applications and is generally indicated by reference numeral 200. When a measurement is performed using this NMR instrument 200, a sample tube 1 holding a sample 30 therein is set within a static magnetic field produced by a magnet 2 whose magnetic field distortion has been corrected by a room-temperature shim set 3 that is under control of a magnetic field corrector 6. RF pulses having a frequency corresponding to the intensity of the static field are applied to the sample 30, thus inducing a nuclear magnetic resonance.
In this experiment, the RF pulses are applied to the sample 30 within the sample tube 1 from the multiple-resonance NMR probe 4. In particular, pulsed signals from an oscillator 14 are selected from plural frequency bands (in the illustrated example, there are 3 bands) according to a nuclide within the sample 30 that is to be observed. The signals are amplified by power amplifiers 13, 15, and 16, respectively, which correspond to frequencies f1, f2, and f3, respectively. The outputs from the power amplifiers 13, 15, and 16 are supplied to the multiple-resonance NMR probe 4 via a duplexer 9 that switches the line between input and output. Thus, the RF pulses are applied to the sample 30.
The sample 30 produces an NMR signal having a resonance frequency intrinsic to the nuclide because of an NMR phenomenon. The NMR signal is picked up by the multiple-resonance NMR probe 4.
Where it is necessary to investigate the sample 30 at a given temperature, the temperature of the vicinities of the sample tube 1 within the multiple-resonance NMR probe 4 is controllably varied by a temperature-varying device 5 that is under control of a computer 7.
The NMR signal picked up by the multiple-resonance NMR probe 4 is fed to an amplifier 10 via the duplexer 9 and amplified. The signal is then converted into an audio frequency by a demodulator 11. The audio frequency is converted into a digital signal by an A/D converter (ADC) 12.
The digital signal is accepted into the computer 7 in this way. The computer 7 analyzes the signal and thus analyzes the sample 30. The results of the analysis are displayed on a display device 8. Consequently, the structure of the substance is investigated by the multiple-resonance NMR spectrometer.
FIG. 2A shows an RF resonant circuit incorporated in NMR probes. This prior-art example is an example of an unbalanced resonant circuit. This resonant circuit has a tuning capacitive element C1 for tuning, a variable capacitor that is an auxiliary tuning variable capacitive element for tuning, and a variable capacitor that is a tuning variable capacitive element for matching. To avoid interference between the tuning matching portion and the sample coil portion, the sample coil portion is electromagnetically shielded from the tuning matching portion by a conductive support that is at ground potential. Two lead wires are brought out from the sample coil portion via two small holes formed in the support. One of the lead wires is connected with a conductive frame, which surrounds the NMR probe and is at ground potential. Since this resonant circuit is an unbalanced circuit, the amplitude of the RF magnetic field maximizes at the upper end of the sample coil and decreases down to zero at the lower end of the sample coil.
The prior-art example of FIG. 2B is an example of balanced resonant circuit. The resonant circuit has tuning capacitive elements C1 and C2 for tuning, a variable capacitor that is an auxiliary tuning variable capacitive element for tuning, and a variable capacitor that is a variable capacitive element for matching. To avoid interference between the tuning matching portion and the sample coil portion, the sample coil portion is electromagnetically shielded from the tuning matching portion by a conductive support that is at ground potential. Two lead wires are brought out from the sample coil portion via two small holes formed in the support. One of the lead wires is connected with the tuning matching circuit. The other lead wire is connected with a conductive frame at ground potential, the frame surrounding the NMR probe via the tuning capacitive element C2. Because the resonant circuit is a balanced circuit, the amplitude of the RF magnetic field maximizes at the upper and lower ends of the sample coil and becomes null at the center of the sample coil.
When the tuning range of this RF resonant circuit is extended, the capacitive element C1 or C2 is removed and replaced with another element, such as a capacitive element, having a different capacitance or an inductive element (such as a coil). This replacement is performed by inserting or withdrawing a stick, or a shaft, having an element attached to its front end (see Japanese Utility-Model Laid-Open No. H2-45477 and Japanese Utility-Model Laid-Open No. H3-10282) instead of by soldering.
FIGS. 3 and 4 show devices each of which is equipped with multiple elements to automate the insertion and withdrawal described above. The device of the type shown in FIG. 3 is applied to an unbalanced resonant circuit, such as prior-art as shown in FIG. 2A. This device is used to replace the tuning capacitive element C1 by another element. The replacing element is disposed on a disk mounted on a rotary shaft. The element C1 is replaced by another element by rotating the disk. This shifts the resonance frequency of the unbalanced resonant circuit, such as prior-art shown in FIG. 2A, thus widening the tuning range (Japanese Patent Laid-Open No. H3-223686).
On the other hand, the device of the type shown in FIG. 4 is applied to a balanced resonant circuit, such as prior-art example 2 of FIG. 2B. This device is used to replace the pair of tuning capacitive elements C1 and C2 by other elements. A replacing pair of elements is disposed on a rectangular slider that is mounted on a rail. The pair of the elements C1 and C2 is replaced by another element pair by causing the slider to slide. This shifts the resonance frequency of the balanced resonant circuit, such as prior-art shown in FIG. 2B, extending the tuning range.
These elements are sufficiently spaced from each other to prevent electric discharging between the elements.
One problem with the multi-element switched NMR probe as described above is that it cannot be applied to a balanced resonant circuit because elements are replaced one by one, for example, in the case of the structure of FIG. 3, though the probe can be applied to an unbalanced resonant circuit. Furthermore, the disk plane is perpendicular to the static magnetic field and so eddy currents are produced within the disk plane during rotation. The resulting local magnetic fields adversely affect the NMR lock. Therefore, when the disk is rotating, limitations occur. That is, the loop of the NMR lock cannot be closed. No measurements can be made. The resolution cannot be altered. As a result, improvement of the throughput of high-speed processing tends to be hindered.
On the other hand, in the case of the structure of FIG. 4, parasitic inductance is induced in the longitudinal direction and so, in this system, the resonance frequency of the switching mechanism itself can be increased only up to about 100 MHz. In a high magnetic field NMR spectrometer of the order of hundreds of MHz, the performance suffers from fatal degradation. In high magnetic field NMR spectroscopy, the resonance frequency of 1H nucleus has today reached a value corresponding to 920 MHz. There is a difference of more than 600 MHz when viewed from lower magnetic field NMR spectroscopy corresponding to 300 MHz. Incidentally, the resonance frequency of 31P nucleus corresponds to 243 MHz in a 600 MHz NMR spectrometer. Where the slider method is implemented, a limitation has been already reached. This leads to deterioration of the performance.
When the slider method is implemented, elements are made to slide on the rail. Therefore, the resistive component of the conductor corresponding to the contact resistance and the total length of the line is increased extremely. This gives rise to a decrease in the Q value of the circuit. That is, the resistance value is hundreds of mΩ to orders of Ω. A Q value of the order of hundreds is inevitably produced. Furthermore, parasitic inductance of from more than ten nH to tens of nH is produced. This produces parasitic resonance interfering with the circuitry for nuclides in the HF region (such as 1H and 19F) that is natural to an NMR detector. Consequently, the performance on the HF side is seriously affected adversely.
More specifically, parasitic inductance of 10 nH and stray capacitance of 4 pF together produce parasitic resonance of about 800 MHz. Parasitic inductance of 10 nH and stray capacitance of 10 pF together produce parasitic resonance of about 500 MHz. Parasitic inductance of 30 nH and stray capacitance of 10 pF together produce parasitic resonance of about 300 MHz. Unwanted signals produced by such parasitic resonances must be eliminated as ghost signals. If such ghost signals increase in number, the operator cannot help neglecting the effects on nuclides other than the main nuclides.
Besides, there are other problems. (i) The slider unavoidably uses a synthetic resin, increasing the background signal. (ii) When each element is made to slide on the rail, electromagnetic noises are produced. (iii) Since the rail is long, wavelength resonance tends to be produced. Incidentally, a quarter wavelength resonance at 800 MHz occurs at about 90 mm. (iv) Electromagnetic radiation from under the NMR detector is promoted.